Variable speed hydraulic drive

ABSTRACT

A hydraulic drive apparatus which is adapted to drive earth drilling apparatus at a plurality of selected speeds and torques in forward and reverse directions. The apparatus includes a double pump operably connected to a pair of hydraulic motors by a hydraulic system. The hydraulic system includes valve means for selecting the volume flow rate of hydraulic fluid to the motors to change their operating speed by allowing one or both pumps to supply hydraulic fluid to the motors. Other valve means are included for selectively changing the hydraulic system flow paths so that the motors are operable in both series and parallel operation so that the speed and torque output is changeable. The hydraulic system is provided with a directional flow control valve which permits changing the direction of hydraulic fluid flow through the motors to change the direction of rotation of the motors and thereby drive a drill string in forward and reverse directions.

United States Patent [191 Snyder [451 Sept. 9, 1975 VARIABLE SPEED HYDRAULIC DRIVE Byron Snyder, Enid, Okla.

[73] Assignee: Azcon Corporation, St, Louis, Mo.

[22] Filed: July 24, 1974 [21] Appl. No: 491,286

[75] Inventor:

Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or FirmFishburn, Gold & Litman [57] ABSTRACT A hydraulic drive apparatus which is adapted to drive earth drilling apparatus at a plurality of selected speeds and torques in forward and reverse directions. The apparatus includes a double pump operably connected to a pair of hydraulic motors by a hydraulic system. The hydraulic system includes valve means for selecting the volume flow rate of hydraulic fluid to the motors to change their operating speed by allowing one or both pumps to supply hydraulic fluid to the motors. Other valve means are included for selectively changing the hydraulic system flow paths so that the motors are operable in both series and parallel operation so that the speed and torque output is changeable. The hydraulic system is provided with a directional flow control valve which permits changing the direction of hydraulic fluid flow through the motors to change the direction of rotation of the motors and thereby drive a drill string in forward and reverse directions.

11 Claims, 8 Drawing Figures VARIABLE SPEED HYDRAULIC DRIVE The principal objects of the present invention are: to provide a hydraulically powered drive apparatus adapted to drive an earth drill at a plurality of speeds and torques with the apparatus having readily accessible controls for operating the apparatus from a remote position; to provide such an apparatus with valve and control means operative in response to the drill encountering a pre-selected resistance for automatically shifting the hydraulic system to a lower speed, higher torque operating condition; to provide such an apparatus with a pair of hydraulic motors to drive the drill bit wherein the motors selectively operate in series or in parallel in the hydraulic system to provide different torque outputs; to provide such an apparatus which requires a minimum of hydraulic lines in the hydraulic system; pump and valve means which are operable to select one or both pumps and thereby the volume flow rate of hydraulic fluid to the motors for effecting different operating speeds of the drill bit; to provide such an apparatus with pressure control means for effecting control of the torque output of the motors and preventing excessive pressure buildup in the hydraulic system; to provide such an apparatus with means for preventing cavitation in the hydraulic system when the hydraulic motors are operating in series; and to provide such an apparatus which has a minimum number of parts, which is easy to maintain and operate and well adapted for its intended use.

Other objects and advantages of the present invention will become apparent from the following description taken in connection with the accompanying drawings wherein are set forth by way of illustration and example certain embodiments of the present invention.

FIG. 1 is an elevational view of an earth drilling apparatus.

FIG. 2 is an enlarged section view of the drilling apparatus taken along the line 22, FIG. 1.

FIG. 3 is a schematic view of the hydraulic fluid supply system illustrating a first operating condition in a forward direction with pressurized supply conduits shown as solid lines and return conduits shown as broken lines.

FIG. 4 is a schematic view of the hydraulic fluid supply system illustrating a second operating condition in the forward direction with pressurized supply conduits shown as solid lines and return conduits shown as broken lines.

FIG. 5 is a schematic view of the hydraulic fluid supply system illustrating a third operating condition in the forward direction with pressurized supply conduits shown as solid lines and return conduits shown as broken lines.

FIG. 6 is a schematic view of the hydraulic fluid supply system illustrating a fourth operating condition in the forward direction with pressurized supply conduits shown as solid lines and return conduits shown as broken lines.

FIG. 7 is a schematic view of the hydraulic fluid supply system illustrating a first operating condition in a reverse direction with pressurized supply conduits shown as solid lines and return conduits shown as broken lines.

FIG. 8 is a schematic view of the hydraulic fluid supply system illustrating a second operating condition in the reverse direction with pressurized supply conduits shown as solid lines and return conduits shown as broken lines.

As required, detailed embodiments of the present invention are disclosed herein however, it is to be understood that the disclosed embodiments are merely exemplary of the invention which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriate detailed structure.

Referring more in details to the drawings:

The reference numeral 1 designates generally an apparatus for drilling or boring holes in the earth. The apparatus 1 includes a conventional derrick or mast 2 which suitably supports in up and down travelling relation a travelling head 3 which has a drill string or stem 4 rotatably supported thereon with power drive means 5 operably connected to the drill string 4 for rotating same and effecting drilling in the earth by a drill bit (not shown) attached to the end of the drill string 4. Guides or ways 7 are secured to the derrick 2 and receive therein torque arms 8 which prevent rotation of the travelling head 3 during operation of the drive means 5. The apparatus 1 is provided with conventional means for raising and lowering the travelling head 3 and as illustrated pull down chains or cables 10 are secured to pull down or crowd links 11 which are operably connected to draw works for applying downward force on the drill string 4 to effect drilling. A pull down yoke 12 is suitably connected to the travelling head 3 and carries therewith a gooseneck swivel l3 and a conventional air or water swivel l4 and a travelling head spindle 15 which is turn connects the gooseneck swivel l3 and water swivel 14 to the drill string 4. Chains or cables 16 are connected to the yoke 12 and are connected to draw works for supplying an upwardly directed force to the travelling head 3 and drill string 4 when the weight of the drill string exceeds the downward drilling force required for drilling. A hoisting block 18 is suitably connected to the yoke 12 and has a hoisting line 19 connected thereto for hoisting the travelling head 3 and drill string 4 upwardly in the derrick 2 for adding additional drill pipe and other service work.

The drive means 5 are operably connected to suitable power means for effecting operation thereof to power drive the drill string 4 preferably at a plurality of selected speeds and torques. In the illustrated structure the drive is hydraulic and the drive means 5 is hydraulic motor means 22 connected to pump means 21 by a hydraulic piping system and valve control system with hydraulic motor means 22 being driven by the pump means 21. Preferably the pump means is a type for selectively having different volume outputs. The hydraulic motor means 22 includes a pair of hydraulic motors 23 and 23' mounted on the travelling head 3 with the hydraulic system and valving arrangement connecting the motors to the pump means and arranged to selectively permit different pressure and volume inputs into the motors 23 and 23' either in series or parallel operating relation to effect different rotational speeds and torque outputs.

In the illustrated structure the pump means 21 is a double fixed displacement pump having pumps 24 and 25 connected to a reservior 27 of hydraulic fluid by a supply line 26. Separate pressure supply lines 28 and 29 are connected to the pumps 24 and 25 respectively with a valve such as a check valve, connected between the lines 28 and 29 so that the pump 24 will not pump fluid into the supply line 29, while permitting flow of hydraulic fluid from the pump 25 into a main supply line 31. Preferably, means are provided for selectively permitting the pump 25 to pump hydraulic fluid back to the reservoir rather than the supply line 31 to reduce the quantity of hydraulic fluid supplied to the motors. As illustrated a pressure relief valve 33 is connected to the line 29 and a return line 34 is connected between the exhaust of the valve 33 and the reservoir 27. Preferably, the valve 33 is of a vented type having a line 35 connected to the vent port with a valve 35', of the manually controlled open/closed type, con nected in the line 35. Opening of the valve 35' releases the venting pressure in the valve 33 which then functions as a substantially free flow valve allowing hydraulic fluid pumped by the pump 25 to be returned to the reservoir 27.

Valve means 36 are connected to the supply line 31 and have hydraulic lines 37 and 38 connected between same and the motors 23 and 23'. A return line 39 is also connected to the valve means 36 to permit the fluid used to operate the motors 23 and 23 to be returned to the reservoir 27 after flowing back through the valve means 36. In the illustrated structure the valve means 36 includes a directional flow control valve 41 such as a four-way three position valve which is selectively operable to permit presurized hydraulic fluid to flow to the motors 23 and 23' in a forward direction or in a re verse direction by selecting one of the lines 37 and 38 to be the supply line thereto. The valve means 36 also includes a pressure relief valve 42 connected between the supply line 31 and the return line 39 and is operable to relieve excessive pressure buildup in the supply line 31 and thereby prevent damage to the hydraulic system and control the maximum torque applied by the motors 23 and 23'.

In the illustrated structure the line 37 is connected to an inlet 44 of the motor 23 and the line 38 is connected to an outlet 45 of the motor 23'. A line 47 is connected to the line 37 having a valve member 48 therebetween which is selectively openable to permit pressurized hydraulic fluid to flow to an inlet 51 of the motor 23. Preferably, the valve 48 is of a pilot operated check valve type having a pilot line 49 connected between the valve 48 and the supply line 28 with a valve 50 connected in the line 49 to selectively permit pilot pressure fluid to flow to the valve 48 for opening same. The motor 23 has an outlet 52 with a line 53 connected thereto and also connected to the return line 38 to provide an exhaust line for the motor 23.

A line 54 connects the outlet 52 to the inlet 51 for se' ries operation of the motors 23 and 23 and has valve means therein which are operable to permit the parallel operation of the motors 23 and 23'. As illustrated the valve means in the line 54 include two check valves 55 and 56 preferably mounted on the travelling head 3 and are in opposed free flow relation with the check valve 55 being free flow from the outlet 52 to the inlet 51 and the valve 56 being free flow in the direction from the inlet 51 to the outlet 52. Preferably the valve 56 is of a pilot operated type having a pilot line 57 connecting same to the line 37 whereby when the line 37 is a sup ply line to the motors 23 and 23' the valve 56 is opened and when the line 37 is a return line the valve 56 is closed preventing exhaust from the motor 23 from flowing into the motor 23'. Flow of the fluid from the line 47 is prevented from entering the line 53 and outlet 52 by the valve 55. Means is connected to the valves 55 and 56 providing a pressure relief between same. As shown the means is a flow restricter 58 which is also connected to the line 53 to insure free operation of the valves 55 and 56 and preventing a pressure buildup therebetween which would prevent same from functioning properly.

Drain means are connected to the motors 23 and 23' so that leakage due to the high pressure hydraulic fluid can be drained therefrom back into the reservoir 27 wherein the drain means includes drain lines 59 connected to each of the motors 23 and 23' and the reservoir 27. Preferably a gauge 60 is provided and is connected to the hydraulic system adjacent the motors to provide a pressure indication of the hydraulic fluid at the motors. As illustrated the gauge 60 is connected to the line 57 to indicate pressure therein. It is to be noted that filters or strainers can be provided in certain of the hydraulic lines such as lines 26, 34 and 39 to help keep the hydraulic fluid clean.

The motors 23 and 23' are operably connected to drive means which in the illustrated structure includes a gear reducing unit 61 connected to each of the motors 23 and 23 and operably connected to driving means such as a gear 62 connected to the drill string 4. As for example, the gear reducer 61 which is illustrated schematically in FIGS. 3 through 8 inclusive can be in the order of a fifteen to one gear reduction.

The present invention is more fully understood by a description of the operation thereof. It is preferred that the apparatus 1 have variable operating speeds and torques and be reversible in direction of rotation of the drill string 4. In the particular form of the invention shown, a plurality of forward speeds and torques as for example four combinations thereof in the forward direction and two combinations thereof in the reverse direction can be obtained. Preferably the valves 33, 36, 41, 42 and 50 or their controls and the gauge 60 are mounted in a drillers station 63 remote from the travelling head 3 for easy control of the apparatus 1 from one position. It is to be noted that the lines 37 and 38 have portions thereof that extend from the drillers station 63 to the travelling head 3 and preferably are flexible to permit restriction free movement of the travelling head 3. It is to be further noted that only two lines are required to extend from the drillers station 63 to the travelling head 3 to accomplish the parallel and series modes of operation of the motors 23 and 23'.

The first operating condition arrangement of the hydraulic system in the forward direction at the highest torque and lowest speed output is illustrated in FIG. 3 wherein the motors 23 and 23' operate in parallel and rotate clockwise. The valve 41 is shifted, preferably manually, so that pressurized hydraulic fluid in the line 31 is supplied to the line 37 with the line 38 communicating with the line 39 which functions as a return line to the reservoir 27. The valve 35' is preferably shifted manually to open same vent the valve 33 whereby the fluid pumped by the pump 25 returns to the reservoir 27 with the check valve 30 preventing the fluid from the pump 24 from being pumped to the reservoir 27. The valve 48 is opened by opening the valve 50 to supply pilot fluid thereto whereby the motors 23 and 23' each receive one-half of the volume of the hydraulic fluid for parallel operation. The exhaust hydraulic fluid is returned through the lines 53, 38 and 39 to the reservoir 27 from the motors 23 and 23'. For example, pump 24 can have an output of 33 gallons per minute at a pressure of 2250 PS1 which is supplied to the valve means 36 and through the lines 37 and 47, to the motors 23 and 23'. It is to be noted that, as described, above, when the line 37 is a supply line the valve 56 is opened by pilot pressure fluid being supplied through the line 57 to the valve 56 which is preferably a pilot operated check valve. With the valve 56 open for parallel operation of the motors the valve 55 remains closed because of a pressure differential thereon, with the supply line 47 having a greater pressure than the pressure in the line 54 at the outlet 52. A second operating condition arrangement having a higher speed and lower torque than the first operating condition arrangement is illustrated schematically in FIG. 4. The valves 33, 35 and 56 are open and the valve 30 is closed and the valve 41 is in the forward direction position. The valve 50 is closed thereby closing the valve 48 so that the aforementioned output of the pump 24 for the first operating condition is supplied only to the motor 23. With the valve 56 open the valve 55 is in a free flow direction from the motor 23 to the motor 23 whereby the motor 23 exhausts through the motor 23' and the motors 23 and 23' drive the drill string 4 through series operation of the motors. It is to be noted that in the series mode of operation of the second operating condition the motors 23 and 23' are both operatively to the drill string 4 by the gear reducers 61 and the gear 62. Driving rotation of the motor 23 will efi'ect following rotation of the motor 23' by their operative connection and due to exhaust flow from motor 23. During operation of hydraulic motors a certain amount of fluid is lost due to leakage and in the illustrated structure the motor 23 could leak a sufficient quantity of hydraulic fluid to prevent a full volume flow of hydraulic fluid from reaching the motor 23 whereby cavitation could result therein. In this regard the return line 53 serves a dual function by providing an anti-cavitation loop whereby hydraulic fluid lost due to the leakage from the motor 23 can be made up or replaced by a portion of the exhaust hydraulic fluid from the motor 23' which is pumped back through the line 53 and the motor 23' which functions as a pump when driven by the exhaust flow from motor 23 during series operation of the motors.

A third forward operating condition arrangement is illustrated in FIG. 5 for providing a lower torque and higher speed than that developed in the first and second operating conditions. The valve 35' is closed whereby the valve 33 functions as a pressure relief valve which for example can be set at 1250 PSI and the valve 30 is in a free flow direction from the pump 25 to the line 31. As described for the first and second operating conditions the valve 41 is in the position for forward rotation. The valve 50 is open and thereby opens the valve 48 to allow flow of fluid into the inlet 51 of motor 23 and the valve 56 is also open which allows parallel operation of the motors 23 and 23'. In the third operating condition the valve 55 is closed due to the pressure differential thereon. As described above, the pumps 24 and 25 each have different volume outputs as for example the pump 25 can have an output of 60 gallons per minute at approximately 1250 PSI and the pump 24 can have an output of approximately 35 gallons per minute at 1250 PSI. Both the pumps supply pressurized fluid to the line 31 through the valves 41 and into the supply line 37 and with the valve 48 open the motors 23 and 23 operate in parallel, each receiving one half of the total output to provide a lower torque and a higher speed than the first or second operating condition. A fourth forward operating condition arrangement to provide the highest speed and lowest torque is illustrated in FIG. 6. The valve 35' is closed whereby the valve 33 functions as a pressure relief valve as in the third operating condition and the valve 30 is in a free flow open condition from the pump 25 to the line 31. The valve 41 is in the position for forward rotation. The valve 50 is closed thereby closing the valve 48 for series operation of the motors 23 and 23'. With the valve 56 being open the valve 55 is in a free flow direction from the motor 23 to the motor 23 to allow exhaust from the motor 23 to flow through the motor 23 as described for the second operating condition illustrated in FIG. 4. In the forth operating condition the motor 23 operates in series with the motor 23' and receives the entire output of the pumps 24 and 25. It is to be noted that if the pressure required to operate drill string 4 in the third and fourth operating condition exceeds the opening pressure set on the valve 33, which is described above as being 1250 PSI, it will open and the check valve 30 will close due to a pressure differential thereon thereby preventing the higher pressure hydraulic fluid in the line 28 from flowing into the line 29. With the valve 33 open the output of the pump 25 will return to the reservoir 27 through line 34 and reduce the quantity of hydraulic fluid supplied to the line 37. Should the pressure exceed that set on the relief valve 33 during the third operating condition the hydraulic system automatically shifts back to the first operating condition. Should the pressure required exceed that on the valve 33 while operating in the fourth condition an automatic shift to the second operating condition will result similar to the shift drive for the third operating condition.

For reverse rotation of the drill string 4 at least two operating conditions can be obtained with the system as shown. A first operating arrangement for reverse rotation is illustrated in FIG. 7 and would be of a low speed and high torque. The valve 35 is open so that the output of the pump 25 is returned to the reservoir 27 through the valve 33 as described above for forward rotation, with the valve 30 being held closed by the pressure differential thereon. The valve 41 is shifted to the position shown in FIG. 7 whereby the fluid supplied from the pump 24 through the line 31 is supplied to the line 38 and the line 37 communicates with the line 39 to function as the return line. The valve 50 and 56 are closed. The pressurized hydraulic fluid is supplied to the outlet 45 of the motor 23' and the outlet 52 of the motor 23 through the lines 38 and 53 for parallel operation of the motors with each receiving one-half of the hydraulic fluid and have their respective exhaust fluid return through the lines 37 and 47. The check valve 55 is open due to a pressure differential thereon and with the valve 56 closed due to the pressure differential thereon the valve 56 prevents flow of hydraulic fluid between the motors 23 and 23'. In reverse operating conditions the valve 48 is in the free flow condition allowing exhaust therethrough into the line 37 from the line 47. A second operating reverse condition is shown in FIG. 8 and is obtained by operating the motors 23 and 23' in parallel. The valve 41 is in the reverse rotation position and the valve 35' is closed whereby the valve 33 functions as a relief valve which as for example is set at 1250 PSI similar to the forward rotation conditions. The valve is in the free flow direction from the line 29 to the line 31 permitting the output of the pump 25 to be supplied to the line 3]. The valve 56 is closed whereby the valves 55 and 56 prevent flow of fluid between the motors 23 and 23 as described in the first reverse operating condition. The valve 50 is closed and the valve 48 is in a free flow condition from the line 47 to the line 37 as described in the first operating condition. The pumps 24 and 25 supply pressurized hydraulic fluid to the line 38 through the valve 41, as for example, the pump 25 can have an output of 60 gallons per minute at 1250 PSI and the pump 24 can have an output of gallons per minute at 1250 PS]. With the motors operating in parallel each receives one-half of the output of the pumps 24 and 25 with the hydraulic fluid being supplied through the lines 38 and 53. Exhaust from the motors 23 and 23' is returned to the reservoir 27 through the lines 47 and 37 to the valve 41 and then through the line 39 to the reservoir.

It is to be noted that the torque output of the motors 23 and 23' is dependent on the pressure of the hydraulic fluid supplied thereto with the maximum pressure being controlled by the valve 42 which is preferably adjustable. [t is to be further noted that different operating conditions can be achieved with a little modification or variation in the hydraulic system described above.

It is to be understood that while I have illustrated and described certain forms of my invention it is not to be limited to the specific form or arrangement of parts herein described and shown.

What I claim and desire to secure by Letters Patent ISI 1. In combination with a drilling apparatus having a drill string and a drill bit connected to the free end of the drill string, a power drive apparatus for rotating the drill string and drill bit, said power apparatus comprising:

a. a source of hydraulic fluid;

b. pump means connected to said source and operable to pump hydraulic fluid therefrom;

c. a drive unit including a plurality of hydraulically operated motor means operative for rotating the drill string;

d. a conduit system connected to said hydraulic motor means and said pump means for conducting pressurized hydraulic fluid from said pump means to said hydraulic motor means, said conduit system including supply conduits each connected to a respective said hydraulic motor means and exhaust conduits each connected to a respective said hydraulic motor means;

e. first valve means connected to one of said supply conduits for selectively permitting hydraulic fluid to flow to said plurality of hydraulic motor means and one of said hydraulic motor means exhaust conduit;

f. second valve means connected between one hydraulic motor means supply conduit and the remaining hydraulic motor means exhaust conduit selectively preventing flow of hydraulic fluid therebetween;

g. said pump means includes first and second pumps having different volume outputs;

h. third valve means connected to said first pump for selectively permitting same to pump hydraulic fluid to a reservoir thereby decreasing the flow rate of pressurized hydraulic fluid pumped to the hydraulic motor means; and

i. a pressure relief valve connected to said conduit system between said pump means and said hydraulic motor means for maintaining pressure below a predetermined level.

2. The drive apparatus as set forth in claim 1 including:

a. fourth valve means connected to said conduit system between said first and second pumps and said supply and exhaust conduits of said hydraulic motor means and selectively controlling the direction of flow of hydraulic fluid to said hydraulic motor means wherein the same are selectively rotatable in forward and reverse directions.

3. The power drive system as set forth in claim 1 wherein said fourth valve means includes:

a. a vented pressure relief valve between said first pump and said reservoir for maintaining the pressure output thereof below a predetermined level;

b. a valve for opening and closing a vent of said vented pressure relief valve for selectively opening said vented relief valve and allowing said first pump to pump hydraulic fluid back to the reservoir; and

c. a check valve between an outlet of said second pump and an outlet of said first pump preventing hydraulic fluid pumped by said second pump from the flowing through said vented relief valve.

4. The power drive system as set forth in claim 3 wherein:

a. said hydraulic motor means includes a pair of hydraulic motors each having an inlet and outlet with said conduit system connected thereto and with said first and second valve means selectively permitting parallel operation thereof; and

b. said conduit system having a conduit portion con nected between the outlet of one of said hydraulic motors and the inlet of the other said hydraulic motor with certain of said second valve means connected thereto for selectively allowing exhausted hydraulic fluid from said one hydraulic motor to be exhausted through the other said hydraulic motor for series operation thereof.

5. The power drive system as set forth in claim 4 including:

a. means associated with at least one of said hydraulic motor means for preventing cavitation therein when said hydraulic motor means are being operated in series; and

b. conduits for conducting hydraulic fluid leakage from said hydraulic motor means to the reservoir.

6. In combination with a drilling apparatus having a drill string and a drill bit connected to the free end of the drill string, a power drive apparatus for rotating the drill string and drill bit, said power apparatus comprising:

a. a drive unit including a pair of hydraulic motor means operative to rotate the drill string;

b. a source of hydraulic fluid;

c. a conduit system connected to said source for conducting hydraulic fluid therefrom to said hydraulic motor means and returning hydraulic fluid thereto;

pump means including a plurality of pumps connected to said conduit system for pressurizing and inducing flow of said hydraulic fluid from said source to said hydraulic motor means;

. first valve means connected to one of said pumps for selectively permitting the output thereof to return to said source ,thereby reducing the volume flow rate of hydraulic fluid to said hydraulic motor means;

. second valve means for directing pressurized hydraulic fluid selectively to one of said hydraulic motor means and both of said hydraulic motor means;

. said first valve means including:

1. a vented pressure relief valve operable to allow said one pump to pump hydraulic fluid back to said source at a predetermined pressure;

a valve connected to a vent of said pressure relief valve for selectively venting same to permit hydraulic fluid to flow through said pressure relief valve at a minimum of pressure; and

a check valve in said conduit system between outlets of said one pump and the other said pump preventing hydraulic fluid from the other pump from flowing through said vented pressure relief valve.

The power drive system as set forth in claim 15 including:

wherein said second valve means includes:

a second valve selectively openable for permitting pressurized hydraulic fluid to flow through both of said hydraulic motor means whereby both hydraulic motor means operate in parallel relation and drive said drill string; and

a pair of check valves between said hydraulic motor means in opposed free flow directions with one of said check valves selectively openable to allow hydraulic fluid exhausted from the one hydraulic motor means to flow through the other hydraulic motor means with said second valve closed for series operation of said hydraulic motor means. The power drive system as set forth in claim 8 including:

the hydraulic fluid from said one hydraulic motor means is exhausted thereinto during series operation.

10. In combination with a drilling apparatus having a drill string and a drill bit connected to the free end of the drill string, a power drive apparatus for rotating the drill string and drill bit, said power apparatus comprising:

a. a source of hydraulic fluid;

b. first and second pumps connected to said source and operable to pump hydraulic fluid therefrom, said first and second pumps having different volume outputs;

c. a drive unit including a plurality of hydraulically operated motor means operative for rotating the drill string;

d. a conduit system connected to said hydraulic motor means and said first and second pumps for conducting pressurized hydraulic fluid from said first and second pumps to said hydraulic motor means, said conduit system including supply conduits each connected to a respective said hydraulic motor means and exhaust conduits each connected to a respective said hydraulic motor means;

. first valve means connected to one of said supply conduits for selectively permitting hydraulic fluid to flow to said plurality of hydraulic motor means and one of said hydraulic motor means exhaust conduit;

. second valve means connected between one hydraulic motor means supply conduit and the remaining hydraulic motor means exhaust conduit selectively preventing flow of hydraulic fluid therebetween;

g. third valve means connected to said first pump for selectively permitting same to pump hydraulic fluid to a reservoir thereby decreasing the flow rate of pressurized hydraulic fluid pumped to the hydraulic motor means.

11. The power drive system as set forth in claim 10 wherein:

a. said hydraulic motor means includes a pair of hydraulic motors each having an inlet and outlet with said conduit system connected thereto and with said first and second valve means selectively permitting parallel operation thereof; and

b. said conduit having a conduit portion connected between the outlet of one of said hydraulic motors and the inlet of the other said hydraulic motor with certain of said second valve means connected thereto for selectively allowing exhausted hydraulic fluid from said one hydraulic motor to be exhausted through the other said hydraulic motor for series operation thereof. 

1. In combination with a drilling apparatus having a drill string and a drill bit connected to the free end of the drill string, a power drive apparatus for rotating the drill string and drill bit, said power apparatus comprising: a. a source of hydraulic fluid; b. pump means connected to said source and operable to pump hydraulic fluid therefrom; c. a drive unit including a plurality of hydraulically operated motor means operative for rotating the drill string; d. a conduit system connected to said hydraulic motor means and said pump means for conducting pressurized hydraulic fluid from said pump means to said hydraulic motor means, said conduit system including supply conduits each connected to a respective said hydraulic motor means and exhaust conduits each connected to a respective said hydraulic motor means; e. first valve means connected to one of said supply conduits for selectively permitting hydraulic fluid to flow to said plurality of hydraulic motor means and one of said hydraulic motor means exhaust conduit; f. second valve means connected between one hydraulic motor means supply conduit and the remaining hydraulic motor means exhaust conduit selectively preventing flow of hydraulic fluid therebetween; g. said pump means includes first and second pumps having different volume outputs; h. third valve means connected to said first pump for selectively permitting same to pump hydraulic fluid to a reservoir thereby decreasing the flow rate of pressurized hydraulic fluid pumped to the hydraulic motor means; and i. a pressure relief valve connected to said conduit system between said pump means and said hydraulic motor means for maintaining pressure below a predetermined level.
 2. The drive apparatus as set forth in claim 1 including: a. fourth valve means connected to said conduit system between said first and second pumps and said supply and exhaust conduits of said hydraulic motor means and selectively controlling the direction of flow of hydraulic fluid to said hydraulic motor means wherein the same are selectively rotatable in forward and reverse directions.
 2. a valve connected to a vent of said pressure relief valve for selectively venting same to permit hydraulic fluid to flow through said pressure relief valve at a minimum of pressure; and
 3. a check valve in said conduit system between outlets of said one pump and the other said pump preventing hydraulic fluid from the other pump from flowing through said vented pressure relief valve.
 3. The power drive system as set forth in claim 1 wherein said fourth valve means includes: a. a vented pressure relief valve between said first pump and said reservoir for maintaining the pressure output thereof below a predetermined level; b. a valve for opening and closing a vent of said vented pressure relief valve for selectively opening said vented relief valve and allowing said first pump to pump hydraulic fluid back to the reservoir; and c. a check valve between an outlet of said second pump and an outlet of said first pump preventing hydraulic fluid pumped by said second pump from the flowing through said vented relief valve.
 4. The power drive system as set forth in claim 3 wherein: a. said hydraulic motor means includes a pair of hydraulic motors each having an inlet and outlet with said conduit system connected thereto and with said first and second valve means selectively permitting parallel operation thereof; and b. said conduit system having a conduit portion connected between the ouTlet of one of said hydraulic motors and the inlet of the other said hydraulic motor with certain of said second valve means connected thereto for selectively allowing exhausted hydraulic fluid from said one hydraulic motor to be exhausted through the other said hydraulic motor for series operation thereof.
 5. The power drive system as set forth in claim 4 including: a. means associated with at least one of said hydraulic motor means for preventing cavitation therein when said hydraulic motor means are being operated in series; and b. conduits for conducting hydraulic fluid leakage from said hydraulic motor means to the reservoir.
 6. In combination with a drilling apparatus having a drill string and a drill bit connected to the free end of the drill string, a power drive apparatus for rotating the drill string and drill bit, said power apparatus comprising: a. a drive unit including a pair of hydraulic motor means operative to rotate the drill string; b. a source of hydraulic fluid; c. a conduit system connected to said source for conducting hydraulic fluid therefrom to said hydraulic motor means and returning hydraulic fluid thereto; d. pump means including a plurality of pumps connected to said conduit system for pressurizing and inducing flow of said hydraulic fluid from said source to said hydraulic motor means; e. first valve means connected to one of said pumps for selectively permitting the output thereof to return to said source thereby reducing the volume flow rate of hydraulic fluid to said hydraulic motor means; f. second valve means for directing pressurized hydraulic fluid selectively to one of said hydraulic motor means and both of said hydraulic motor means; g. said first valve means including:
 7. The power drive system as set forth in claim 15 including: a. a directional flow control valve in said conduit system between said pumps and said hydraulic motor means for selectively reversing the flow of hydraulic fluid through said hydraulic motor means and reversing the direction of rotation thereof and of said drill string; and b. a second pressure relief valve in said conduit system between said pumps and said hydraulic motor means for limiting the maximum pressure at which hydraulic fluid is supplied to the hydraulic motor means.
 8. The power drive system as set forth in claim 7 wherein said second valve means includes: a. a second valve selectively openable for permitting pressurized hydraulic fluid to flow through both of said hydraulic motor means whereby both hydraulic motor means operate in parallel relation and drive said drill string; and b. a pair of check valves between said hydraulic motor means in opposed free flow directions with one of said check valves selectively openable to allow hydraulic fluid exhausted from the one hydraulic motor means to flow through the other hydraulic motor means with said second valve closed for series operation of said hydraulic motor means.
 9. The power drive system as set forth in claim 8 including: a. means in said conduit system for preventing cavitation in the other said hydraulic motor means when the hydraulic fluid from said one hydraulic motor means is exhausted thereinto during series operation.
 10. In combination with a drilling apparatus having a drill string and a drill bit connected to the free end of the drill string, a power drive apparatus for roTating the drill string and drill bit, said power apparatus comprising: a. a source of hydraulic fluid; b. first and second pumps connected to said source and operable to pump hydraulic fluid therefrom, said first and second pumps having different volume outputs; c. a drive unit including a plurality of hydraulically operated motor means operative for rotating the drill string; d. a conduit system connected to said hydraulic motor means and said first and second pumps for conducting pressurized hydraulic fluid from said first and second pumps to said hydraulic motor means, said conduit system including supply conduits each connected to a respective said hydraulic motor means and exhaust conduits each connected to a respective said hydraulic motor means; e. first valve means connected to one of said supply conduits for selectively permitting hydraulic fluid to flow to said plurality of hydraulic motor means and one of said hydraulic motor means exhaust conduit; f. second valve means connected between one hydraulic motor means supply conduit and the remaining hydraulic motor means exhaust conduit selectively preventing flow of hydraulic fluid therebetween; g. third valve means connected to said first pump for selectively permitting same to pump hydraulic fluid to a reservoir thereby decreasing the flow rate of pressurized hydraulic fluid pumped to the hydraulic motor means.
 11. The power drive system as set forth in claim 10 wherein: a. said hydraulic motor means includes a pair of hydraulic motors each having an inlet and outlet with said conduit system connected thereto and with said first and second valve means selectively permitting parallel operation thereof; and b. said conduit having a conduit portion connected between the outlet of one of said hydraulic motors and the inlet of the other said hydraulic motor with certain of said second valve means connected thereto for selectively allowing exhausted hydraulic fluid from said one hydraulic motor to be exhausted through the other said hydraulic motor for series operation thereof. 